The present invention relates to modified epoxy resins and the use thereof in resinous blends useful as water-based coating compositions particularly suitable for can or coil coatings.
In the area of can coatings in particular, coatings intended for use in food and beverage industries generally are expected to meet a number of requirements in order to be commercially acceptable. The coating should adhere well to the base metal and should possess flexibility, extensibility and adhesion characteristics so as to withstand the processing of the container itself. The coating sometimes also must be able to resist heat which can be encountered during processing of the container and its contents. Additionally, the coating itself should not affect the taste of a food or beverage which is put into the coated container. Film continuity is another characteristic sought after, and one aspect of this requirement is that coatings be blister-free. Blistering is a defect that arises from gas by-products of curing the coating becoming trapped in the coating, and is a problem particularly associated with coated areas in which the coating is relatively thick. A coating that is prone to blistering requires special precautions to be taken during coating to assure that a maximum allowable coating thickness is not exceeded on any portion of the article being coated. In some cases line speed is limited by a tendency for a coating to blister. Another defect that is preferably avoided is blush, which is a haziness in the film believed to be caused by absorption of water. Blush is particularly evident with container coatings that are subjected to high temperature, high humidity conditions during a camping process.
The prior an typically used substantial excess amounts of amine or ammonia to defunctionalize epoxy resins intended for use in water dispersible compositions of the type involved here for the sake of maintaining relatively low molecular weight. It had been believed that using equivalent ratios close to 1:1 involved a risk of gelling the resin, rendering it useless for coating purposes. This approach, however, required costly collection arrangements to prevent discharge to the atmosphere when the excess amine or ammonia was subsequently driven from the defunctionalized product.
U.S. Pat. No. 4,605,476 (Hart et al.) discloses waterborne can coatings that comprise epoxy resins defunctionalized with an excess of ammonia or amine blended with acrylic copolymers that may incorporate N-(alkoxymethyl) acrylamide or methacrylamide. It would be desirable to utilize less ammonia or amine than employed in the approach of that patent. It would also be desirable to achieve improvements in process resistance in this type of coating.
U.S. Pat. No. 4,174,333 (Hartman et al.) discloses waterborne can coatings containing epoxy resins that have been defunctionalized with excess ammonia or amine and have been reacted with an anhydride. The same improvements would be desirable as mentioned with the previous patent.